Retry boolean CUT that OCCT reports done but subtracts no volume (#5630)

An IfcOpeningElement voiding a dense tessellated first operand (an
IfcGeographicElement terrain built from an IfcPolygonalFaceSet) is not
cut: the opening solid overlaps the terrain, yet the result keeps the
full volume. OCCT BRepAlgoAPI_Cut returns IsDone at the tight default
fuzzy value but subtracts essentially nothing, and because it reports
success the existing escalating-fuzziness retry never runs. At a higher
fuzziness (still well under the min-edge ceiling) the section is computed
correctly.

After a CUT is deemed valid, and only while a higher-fuzziness retry is
still permitted, compare the result volume to the first operand. If the
removed fraction is negligible (< 1e-4 of operand A) treat the CUT as a
failure so the escalation runs and finds a fuzzy value at which the cut
actually happens. When no retry is left the result is accepted as before,
so a genuinely tiny legitimate cut can never lose its outcome. New
diagnostic GEO 156.

Verified on OCC 7.9.2: the terrain goes from 7330.878 (no hole, 459 verts)
to 6058.593 (real hole, 377 verts, still a closed manifold), removing the
expected 1272 wedge. Regression: five currently-passing cut models
(#619, #4118, #5473, #5186, #511) are byte-identical, and across the full
1447-object source file exactly one product (the terrain) changes.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/ifcgeom/kernels/opencascade/boolean_utils.cpp

@@ -1218,6 +1218,24 @@ bool IfcGeom::util::boolean_operation(const boolean_settings& settings, const To
 				}
 			}
 
+			// #5630 A solid-solid CUT whose tool operands survived the disjoint/
+			// touching/narrow elimination above is expected to remove material. On
+			// dense tessellated first operands (e.g. IfcPolygonalFaceSet terrain)
+			// OCCT's BOP can silently return an essentially unchanged result at a
+			// tight fuzzy value, reporting IsDone yet subtracting ~nothing. Detect
+			// this near zero volume removal and, while a higher fuzziness retry is
+			// still permitted, treat it as a failure so the existing escalation can
+			// find a fuzzy value at which the section is actually computed. When no
+			// retry is left the result is accepted as is, preserving prior behaviour.
+			if (success && op == BOPAlgo_CUT && allow_retry) {
+				const double va = shape_volume(a);
+				const double vr = shape_volume(r);
+				if (va > 1.e-9 && (va - vr) < va * 1.e-4) {
+					success = false;
+					Logger::Root().Notice("GEO", 156, "Boolean subtraction removed no volume, retrying with higher fuzziness");
+				}
+			}
+
 			if (success) {
 
 				{